Hydrophilic polymers, such as polyethylene glycol (PEG), have been used for modification of various substrates, such as polypeptides, drugs and liposomes, in order to reduce immunogenicity of the substrate and/or to improve its blood circulation lifetime (Zalipsky & Harris, 1997). For example, parenterally administered proteins can be immunogenic and may have a short pharmacological half-life. Some proteins can also be relatively water insoluble. Consequently, it can be difficult to achieve therapeutically useful blood levels of the proteins in patients.
Conjugation of hydrophilic polymers, particularly PEG (Zalipsky & Harris, 1997), to proteins has been described as an approach to overcoming these difficulties. For example, Davis et al., in U.S. Pat. No. 4,179,337, describe the conjugation of PEG to proteins such as enzymes and insulin to form PEG-protein conjugates having less immunogenicity yet retaining a substantial proportion of physiological activity. Veronese et al. (Applied Biochem. and Biotech, 11: 141-152 (1985)) describe activating polyethylene glycols with phenyl chloroformates for conjugation to a ribonuclease and a superoxide dismutase, respectively. Katre et al., in U.S. Pat. Nos. 4,766,106 and 4,917,888, describe solubilizing proteins by polymer conjugation. U.S. Pat. No. 4,902,502 (Nitecki et al.) and PCT Pubn. No. WO 90/13540 (Enzon, Inc.) describe conjugation of PEG and other polymers to recombinant proteins to reduce immunogenicity and increase half-life.
PEG has also been described for use in improving the blood circulation lifetime of liposomes (U.S. Pat. No. 5,103,556). The PEG polymer is covalently attached to the polar head group of a lipid in order to mask or shield the liposomes from being recognized and removed by the reticuloendothelial system.
Various conjugation chemistries for attachment of PEG to biologically relevant molecules have been reviewed (Zalipsky, 1995a).